1. Field of the Invention
This invention relates to a resin composition, film or laminate excellent in gas barrier properties and to a process for producing the same.
2. Description of the Related Art
Many functions are required for packaging, including adequate gas barrier properties for protecting the package contents. Gas barrier properties are important properties which affect the preservability of foods. Change in distributive systems, diversification of packaging techniques, additive regulations and change in taste increase more and more the necessity for adequate gas barrier properties. However, the gas barrier properties have been weak points of general plastics.
The food-deteriorating factors are oxygen, light, heat, water and the like, and in particular, oxygen is a serious factor. Gas barrier materials are indispensable for effectively intercepting oxygen and simultaneously for controlling the deterioration of foods such as gas-filling, vacuum packaging and the like. Since the barrier materials have a barrier function to not only oxygen gas but also other various gases, organic solvent vapors, fragrance and the like, they can be used for anticorrosion, deodorization and antisublimation and have been utilized in many fields of foods, cosmetics, agricultural chemicals, pharmaceuticals and the like such as confectionery, bonito pack, retort pouch, mineral water container and the like.
Of films consisting of a thermoplastic resin, oriented films of polypropylene, polyester, polyamide or the like have excellent mechanical properties, heat resistance, transparency and the like and are widely used as packaging materials. However, when these films are used for packaging foods, they are unsatisfactory in barrier properties to oxygen and other gases, and therefore, they tend to cause such various problems insofar as their contents become deteriorated by oxidative degradation or with aerobic bacteria, fragrant components permeate the films to lose the taste of the contents, and the moisture in the atmosphere wets the contents to make the taste bad. Thus, usually, such a measure is often taken that other film layers having good gas barrier properties are laminated thereto.
Heretofore, various transparent plastic materials having unsatisfactory gas barrier properties have been known and there are, for example, films composed of polyvinyl alcohol, ethylene-vinyl alcohol copolymer or polyvinylidene dichloride resin. However, these plastic materials are oxygen-permeable in such a degree that the amount of oxygen permeated cannot be neglected while a metal or glass material to be used in canning or bottling has an oxygen permeability of substantially zero.
Another method for imparting gas barrier properties is a method of dispersing an inorganic material in the flat form in a resin. For example, Japanese Patent Application Kokai No. 62-148,532 describes a production method which comprises coating on a releasable substrate a coating composition consisting of 100 parts by weight of a polyurethane resin solution having a concentration of 30% in 1,6-hexanepolycarbonate diol, 25 parts by weight of mica powder and 60 parts by weight of dimethyl-formamide, drying the same and then releasing it from the substrate.
Japanese Patent Application Kokai No. 64-043,554 describes a method of obtaining a film which comprises adding mica particles having an average length of 7 .mu.m and an aspect ratio of 140 to a methanolic aqueous solution of an ethylene-vinyl alcohol copolymer, pouring the solution in a cold water to form a precipitate, removing the precipitate by filtration, drying it to form pellets and then forming the pellets into a film.
Further, Japanese Patent Application Kokai No. 3-93,542 describes a method which comprises coating a biaxially oriented polyethylene terephthalate (OPET) film with a coating composition consisting of a silyl group-containing modified polyvinyl alcohol and synthetic hectorite at a weight ratio of 50:50, drying the same and then heat-treating it at 130-150.degree. C.
However, the films obtained by these techniques are still not sufficient in gas barrier properties and cannot always be said to be satisfactory.